The long-term goal of the project is to more explicitly describe the role of the parathyroid gland in calcium homeostasis. These studies are designed to precisely define the control of parathyroid hormone (PTH) secretion during normocalcemia. PTH secretion rate will be assessed in calves both directly and indirectly. Direct measurement of PTH secretion rate will be accomplished by collection and radioimmunoassay of parathyroid venous blood, while indirect assessments will be made by measurement of plasma immunoreactive parathyroid hormone (iPTH) concentration in instances where this technique is more appropriate to the conduction of long-term studies. Particular emphasis will be placed on evaluating the effects on the parathyroids of other substances (e.g., epinephrine, vitamin D metabolites, and calcitonin) which may modulate the response of the glands to calcium. These studies should determine the relative importance of calcium and other secretagogues in the control of PTH secretion in normocalcemic animals and may provide insight as to mechanisms involved in the development of parathyroid hyperactivity. The presence of immunoreactive hormone fragments in the peripheral circulation has created uncertainty regarding the validity of assessment of parathyroid secretory activity by measurement of plasma iPTH concentrations. Work of other investigators as well as our own indicates that these hormonal fragments arise both by glandular secretion as well as by cleavage on intact hormone in the peripheral circulation. We will attempt to define the relationship between fragment secretory rate and plasma fragment concentration during induced variations of secretory activity. Hopefully, these studies will improve the reliability of plasma iPTH concentration in the assessment of parathyroid secretory activity.